210 C. Davison — On Enrthquahe- Sounds. 



of the sound to the shock is definitely stated. Tliere are in all 423 

 records. The sound is said to have 



Preceded the shock in 



Accompanied or attended it in 



Followed it in 



Preceded and accompanied it in 



Accompanied and followed it in ... 



Preceded, accompanied and followed it in ... 



It must be admitted that the phrase " accompanied " or " attended," 

 singly, is a very vague one. As a general rule, it only means that 

 the sound was heard at about the same time that the shock was felt ; 

 hardly ever, perhaps, that the beginning and end of both sound and 

 shock were coincident. It cannot, then, be taken to exclude cases 

 in which the sound may have overlapped the shock at either end or 

 both. The meaning of the terms " preceded " and " followed " is 

 less ambiguous, though not free from doubt. But, so far as regards 

 the earthquakes recorded by Mallet, it is clear that the beginning 

 of the sound must have preceded that of the sensible shock much 

 more frequently than the end of the sound followed that of the 

 shock. 



Turning, however, from these earthquakes, which are, as a rule, 

 of considerable intensity, to shocks of slight intensity and shdrt 

 duration — shocks, for example, like those generally felt in this 

 country — it will be found that the comparative rarity of subsequent 

 sounds is not so strongly marked. In studying the Inverness earth- 

 quake of November 15, 1890, I received definite replies to the 

 question on the time-relations of the shock and sound from 64 places, 

 with the following results.^ The sound is said to have 



Preceded the shock at 



Accompanied it at ... 



Followed it at 



Preceded and accompanied it at ... 



Accompanied and followed it at 



Preceded, accompanied and followed it at... 



In the other British earthquakes which I have studied the obser- 

 vations on this point are less numerous, but they are sufficient to 

 show that, in certain parts of the disturbed area at any rate, the 

 sound frequently continues to be heard after the close of the sensible 

 shock. 



2. With regard to the Maximum, Intensity of the Shock and Sound. — 

 It has been already remarked that, in British earthtpiakes, the sound 

 increases in intensity, to a maximum, and then dies away. Now, 

 it appears, from a large number of observations, that it is just at the 

 moment when the sound is loudest that the 'principal, vibrations are felt. 



This fact was noticed fifty years ago by Mr. David Milne (after- 

 wards Milne-Home) in a valuable series of papers on the earthquakes 

 of this country. In summarizing their principal features, he i-emarks 

 that there appear to be two distinct sensations, a tremor and a violent 

 blow or concussion, the latter known in Comrie and the neighbour- 

 hood as the " thud." " The tremulous or trembling motion," he says, 

 » Quart. Journ. Geol. Soc. 1891, p. 623. 



